nvme: Use controller's page size instead of PAGE_SIZE to create qpair

When constructing qpair, use the controller's notion of page size rather
than the host's PAGE_SIZE. Currently, these are both 4k, but the arm 16k
page size support requires decoupling.

There's a "hidden" PAGE_SIZE in btoc, so we must change btoc(x) to
howmany(x, ctrlr->page_size) to properly count the number of pages (in
the drive's world view) are needed for various calculations.

With these changes, we the nvme driver operates at production level load
for both host 4k and host 16k page size.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D34873